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The present invention relates to occupancy sensors.
An occupancy sensor is designed to detect the presence of a person(s) in a room, usually in order to determine whether various electrically powered loads in that room (for example, lights, ventilation, and the like) should be turned on or not. This is of particular advantage to institutions that have visitors who are not directly responsible for paying for the electricity they consume, since these people often do not exercise diligence in regularly turning off electrically powered loads, such as lights, ventilation, and the like, when they leave a room. Occupancy sensors may therefore conserve a great deal of energy. This has led many businesses to purchase them voluntarily; it has also resulted in laws in certain states mandating the use of occupancy sensors in large areas as an environmental conservation measure.
The two most prevalent types of occupancy sensors are passive infrared and active ultrasonic devices. A passive infrared ("PIR") sensor will turn on the load whenever it detects a moving or newly apparent heat source. An active ultrasonic sensor emits vibrations at frequencies of 25 kHz or higher and listens to the return echoes; if it detects a significant Doppler shift, indicating the presence of a moving body, then it turns the load on. Either detector will turn the load back off after a certain interval of no motion sensed, usually three to sixty minutes as determined by the user. The motion sensitivity of the device is usually also set by the user.
Each of these sensors is not without disadvantage. PIR sensors may not be able to sense motion behind a barrier within a room, such as a modesty wall in a restroom. In addition, PIR sensors may be susceptible to "dead spots" in certain rooms, within which they are less sensitive to heat sources.
Ultrasonic sensors do not share these handicaps, since the sonic energy they emit will fill the room. However, they may be susceptible to false triggering from several sources. For example, an ultrasonic sensor placed on the wall next to a door may send some sonic energy into the corridor outside the room, and motion within the corridor may make the sensor react as though there were motion in the room resulting in a sensing error. In addition, air motion from wind and ventilation may trip an ultrasonic sensor by mistake. This difficulty is often exacerbated by the user setting the sensitivity of the ultrasonic sensor too high which can increase susceptibility to false tripping.